Anisotropic distribution of the segregated atoms between the two fracture surfaces

Scripta M E T A L L U R G I C A et M A T E R I A L I A

Vol. 25, pp. 955-938, 1991 Printed in the U.S.A.

Pergamon Press plc All rights reserved

A N I S O T R O P I C D I S T R I B U T I O N OF THE S E G R E G A T E D A T O M S B E T W E E N THE TWO FRACTURE SURFACES

M. M e n y h a r d " and C.J. M c M a h o n Jr. ÷ " R e s e a r c h Institute for T e c h n i c a l Physics of the HAS, H-1325 Budapest P.O.Box 76, Hungary, ~ D e p a r t m e n t of M a t e r i a l s Science and Engineering, U n i v e r s i t y of Pennsylvania, Philadelphia, PA 19104-6272 (Received December 17, 1990) (Revised February 15, 1991) In~rmd~

ion

Many i n t e r e s t i n g and s o m e t i m e s harmful p r o c e s s e s like temper embrittlement, stress c o r r o s i o n cracking, grain b o u n d a r y diffusion, etc., are g o v e r n e d or at least s t r o n g l y i n f l u e n c e d by grain b o u n d a r y s e g r e g a t i o n (GBS): thus, its study is of great t e c h n i c a l importance. Auger f r a c t o g r a p h y has been the most important m e t h o d for the e x p e r i m e n t a l a p p r o a c h of s t u d y i n g GBS. By u s i n g this method, one p r o d u c e s a brittle specimen. This s p e c i m e n is to be fractured in situ in the Auger E l e c t r o n S p e c t r o s c o p e (AES). The fracture surface, s u p p o s e d to be the grain b o u n d a r y (GB), is a n a l y z e d by m e a n s of AES. It is g e n e r a l l y s u p p o s e d that the bonds on the two sides of the grain b o u n d a r y plane are b r o k e n s t a t i s t i c a l l y equally, resulting, on a m a c r o s c o p i c scale, in an even d i s t r i b u t i o n of the s e g r e g a t e d atoms on both newly c r e a t e d surfaces. Thus, the c o v e r a g e m e a s u r e d on one of the fracture surfaces is to be m u l t i p l i e d by two to get the GB coverage. It has been known for a long time that the c o v e r a g e of the s e g r e g a t e d atoms on the fracture surface m i g h t show strong v a r i a b i l i t y [1-5]. The number of these findings increases w i t h the number of Auger M i c r o p r o b e s available w i t h high spatial resolution. The e s s e n t i a l q u e s t i o n in this s i t u a t i o n is w h e t h e r or not the v a r i a b i l i t y o b s e r v e d on the fracture surface reflects the v a r i a b i l i t y of the s e g r e g a t e d element on the c o r r e s p o n d i n g GB-s? It was shown that single crystal effects (practically all measurements, using Auger Microprobes, are done on a given facet, that is on a single crystal) can result in a v a r i a b i l i t y of the m e a s u r e d Auger signal even in the case of h o m o g e n e o u s coverage [6-7]. There are rough e s t i m a t e s on the size of this "artifact" [3,8]. An a d d i t i o n a l source of the v a r i a b i l i t y is the uneven d i s t r i b u t i o n of the s e g r e g a t e d atoms during the fracture process. This o b v i o u s l y results in a real p h y s i c a l v a r i a b i l i t y which, on the other hand, does not reflect the original v a r i a b i l i t y of the c o v e r a g e s of the segregated e l e m e n t s on the GB-s. The p o s s i b i l i t y of the u n e v e n d i s t r i b u t i o n of the s e g r e g a t e d atoms during the fracture process was c o n s i d e r e d a long time ago [9], w h i c h is not s u r p r i s i n g from a theoretical point of view. In fact the a t o m i s t i c m o d e l l i n g of the GB has p r o v e d that the GB is a w e l l o r g a n i z e d region, h a v i n g a well d e f i n e d structure. The d i f f e r e n t GB s t r u c t u r e s show d i f f e r e n t c a p a b i l i t i e s to accept s e g r e g a t i n g atoms, r e s u l t i n g in v a r i o u s c o v e r a g e s and atomic a r r a n g e m e n t s on v a r i o u s GB-s. The p o s i t i o n s of the s e g r e g a t e d atoms are not n e c e s s a r i l y symmetrical w i t h respect to the two grains: thus, there is no good reason to suppose that the fracture always d i s t r i b u t e s the s e g r e g a t e d a t o m s evenly b e t w e e n the fracture surfaces. Only a few w o r k s are known, where both sides of the fracture w e r e measured. Suzuki et al. [9] m e a s u r e d the level of s e g r e g a t i o n on one side of the fracture of a Fe-P alloy, and after it, the s t r u c t u r e of the studied facets

935 0036-9748/91 $3.00 + .00 C o p y r i g h t (c) 1991 P e r g a m o n Press plc

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w e r e d e t e r m i n e d by s e l e c t e d area c h a n n e l i n g p a t t e r n technique. T h e y ensured that the v a r i a b i l i t y is c o n n e c t e d to GB structure; the high index GB-s c o n t a i n e d more s e g r e g a t e d p h o s p h o r u s than the low index ones. A l t h o u g h the other side of the fracture was not m e a s u r e d they c o n c l u d e d that the d i s t r i b u t i o n of p h o s p h o r u s was not even b e t w e e n the two sides. T h i s w o r k was, however, c r i t i c i z e d later b e c a u s e the c h a n n e l i n g e f f e c t s were ignored. Briant [3] m e a s u r e d both sides of the f r a c t u r e of iron b a s e d alloys. He found some d i f f e r e n c e s b e t w e e n the p e a k - h e i g h t - r a t i o s (PHR-s) of the two sides of the fracture, but the d i f f e r e n c e s were in the range of 20-25%, and he c o n c l u d e d that the c o v e r a g e on the two sides were e s s e n t i a l l y the same. Somewhat less d i f f e r e n c e was found by H a n s e l et al in ferritic iron [10]. M e n y h a r d et al m e a s u r e d the two sides of f r a c t u r e of a Fe-Si-P s y m m e t r i c a l b i c r y s t a l [11-12] and found a s t r o n g v a r i a b i l i t y of the p h o s p h o r u s coverage on the fracture surface. It turned out, however, that the sum of the c o v e r a g e s m e a s u r e d on the two sides was a c o n s t an t value, showing the u n i f o r m coverage of the p h o s p h o r u s on the GB. In this w o r k we will report on studies c a r r i e d out on both sides of the fracture of a Fe-P alloy. ~t.al_mr~c.e.dur_e The F e - 0 . 0 6 % P alloy was first d e c a r b u r i z e d by a n n e a l i n g in dry h y d r o g e n at 780 ° C for 3 days. A f t e r d e c a r b u r i z a t i o n , the s p e c i m e n s were s e a l e d in e v a c u a t e d quartz c a p s u l e s for a d d i t i o n a l heat t r e a t m e n t at 500 ° C for 500 hours. This p r o c e d u r e r e s u l t e d in a s p e c i m e n s h o w i n g e q u i l i b r i u m or close to e q u i l i b r i u m segregation. The d e t a i l s of this e x p e r i m e n t can be found in [13]. The n o t c h e d s p e c i m e n was f r a c t u r e d in a S c a n n i n g A u g e r M i c r o s c o p e (PHI 600) at 100 K in such a w a y that both sides of the f r a c t u r e could be measured. The part that remained in the sample holder is r e f e r r e d to in the f o l l o w i n g as the down part, the other one as the up part. The fracture was almost c o m p l e t e l y intergranular. Except for iron, p h o s p h o r u s was the only element on the f r a c t u r e surface. After the m a t c h i n g grain b o u n d a r y facets were identified, Auger s p e c t r a were r e c o r d e d in 5-5 points on 16 facets on both sides of the fracture. There was no attempt made to measure the c o n c e n t r a t i o n s in m a t c h i n g p o i n t s as the h o m o g e n e i t y w i t h i n one facet was r e a s o n a b l e good: thus, any point of the facet c o u l d be used to c h a r a c t e r i z e the whole facet. Then the s p e c i m e n was h e a t e d up to 500 ° C and the c o v e r a g e p r o d u c e d by free surface s e g r e g a t i o n (quasi e q u i l i b r i u m was reached after 20-50 minutes) was m e a s u r e d on the down Table i: Average peak height part at the same points w h e r e the ratios fracture surface a n a l y s i s was c a r r i e d down out. The P coverage was c a l c u l a t e d by 7.8 1 10.6 29.8 the e x p r e s s i o n given by Erhart et al 2 14.9 11.7 25.9 [14] and is given always in 3 13.1 9.4 26.4 p e r c e n t a g e of a monolayer. The mean 4 8.5 12.6 28.2 value and the s t a n d a r d d e v i a t i o n 5 12 7 8.5 27.5 (which has no p h y s i c a l meaning, see 14.7 31.0 6 9 6 [13]) will always be c a l c u l a t e d to 7 ii 0 18.1 34.7 give the center and w i d t h of the 8 7 8 10.6 31.4 distribution. 9 0 9 19.0 34.0

As was r e p o r t e d earlier [13], the p h o s p h o r u s c o v e r a g e w i t h i n one facet w a s always homogeneous. In contrast, the c o v e r a g e on d i f f e r e n t facets showed a great variability. Table I. shows the average P H R - s of P(120 eV)/ Fe (703 eV), m e a s u r e d on the down

10 ii 12 13 14 15 18

Mean Std.dev(%)

9 15 12 10 5 16 6

4 8 2 1 3 5 4

13.3 12.6 21.8 12.6 10.6 7.3 4.4

37.0 24.8 40.8 28.2 32.1 28.8 29.8

10.6 30.1

12.4 34.8

30.5 13.8

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(second column) and up (third column) sides of 16 facets. The fourth c o l u m n c o n t a i n s the c o v e r a g e v a l u e s e s t a b l i s h e d by the free surface segregation. In the last two rows the mean v a l u e s and the s t a n d a r d d e v i a t i o n s in p e r c e n t a g e (with respect to the average value) are given. D J h @ ~ ~ ioJs Table i d e m o n s t r a t e s that we have the usual v e r y wide d i s t r i b u t i o n s of the c o v e r a g e s on both surfaces. Also, c l e a r l y there is no simple r e l a t i o n s h i p b e t w e e n the v a l u e s m e a s u r e d on the m a t c h i n g sides; s o m e t i m e s the up side has h i g h e r values, s o m e t i m e s the down side. The d i f f e r e n c e is s o m e t i m e s small (facet #16) and s o m e t i m e s huge (facet #9). On the other hand, the mean v a l u e s c a l c u l a t e d for the 16 facets are rather close, s h o w i n g that from a m a c r o s c o p i c point of v i e w the two s u r f a c e s are similar, as we expected. C o n s i d e r i n g the large d i f f e r e n c e s between the up and down sides of m a t c h i n g facets, one can safely c o n c l u d e that the fracture does not d i s t r i b u t e the p h o s p h o r u s e v e n l y b e t w e e n the two newly c r e a t e d s u r f a c e s and thus the v a r i a b i l i t y of the c o v e r a g e of the s e g r e g a t e d element on the f r a c t u r e surface is larger than that on the GB. To support this c o n c l u s i o n we should, however, c h e c k that no a r t i f a c t s are involved in the m e a s u r e m e n t . It was d i s c u s s e d already that the o b s e r v e d d i s t r i b u t i o n s might have formed b e c a u s e of several reasons such as the f o l l o w i n g : i. v a r y i n g GB coverage, 2. u n i f o r m GB c o v e r a g e but u n e v e n d i s t r i b u t i o n of atoms b e t w e e n the two sides of the fracture in the fracture process, 3. c h a n n e l i n g 4. surface roughness, etc. In a p r e v i o u s p a p e r we gave a rough general e s t i m a t e for the c o m b i n e d e f f e c t s of r e a s o n s 3 and 4 as b e i n g 30-40%, w h i c h m e a n s that any d i f f e r e n c e between two c o v e r a g e s e x c e e d i n g this should be c o n s i d e r e d as a real p h y s i c a l difference. We see that in T a b l e 1 there are several pairs m e e t i n g this requirement. We should recognize, however, that the limit given there was a rough upper limit. To get a better value for the present system, free surface s e g r e g a t i o n e x p e r i m e n t s were c a r r i e d out. W i t h the help of the free surface s e g r e g a t i o n e x p e r i m e n t we can give a realistic upper limit of the e f f e c t s of the c h a n n e l i n g and surface roughne3s in the case of these surfaces. We s u p p o s e d that the heat treatment, a p p l i e d to e s t a b l i s h the free surface segregation, was not s u f f i c i e n t to create s u b s t a n t i a l d i s l o c a t i o n movement: thus, we did not await e s s e n t i a l c h a n g e s in the s t r u c t u r e of the matrix. Thus, we did not expect a change of c h a n n e l i n g b e c a u s e of the heat treatment. If there still is a change, it should arise from the i m p r o v e m e n t of the structure, w h i c h results in a s t r o n g e r c h a n n e l i n g effect. Thus, we conclude that c h a n n e l i n g must have a same or larger effect after the free surface segregation. Surface r o u g h n e s s as a p h y s i c a l p h e n o m e n o n is not very well understeod. O b s e r v a t i o n s showed that the Auger i n t e n s i t i e s d e p e n d on the q u a l i t y of the surface [15,16]. G e o m e t r i c a l i r r e g u l a r i t i e s are thought to produce an a s s e m b l y of small surfaces, with v a r i o u s surface normals, and the intensity is to be c a l c u l a t e d as an average of that assembly. A d o p t i n g this idea. we can use similar a r g u m e n t s as in the case of c h a n n e l i n g to e s t i m a t e the effect of heat t r e a t m e n t on surface roughness. M a c r o s c o p i c c h a n g e s of the surface are not e x p e c t e d even c o n s i d e r i n g very fast surface diffusion. If, in addition, the surface defects also c o n t r i b u t e to the e f f e c t s a t t r i b u t e d to the surface roughness, the heating may heal them and the effect of the surface r o u g h n e s s may decrease. We suppose that by doing this e x p e r i m e n t we compare the surface formed by fracture and the surface p r o d u c e d by free surface s e g r e g a t i o n k e e p i n g the c h a n n e l i n g and surface r o u g h n e s s a p p r o x i m a t e l y constant. The relative w i d t h of the d i s t r i b u t i o n of the c o v e r a g e s of p h o s p h o r u s layer formed by free surface s e g r e g a t i o n is much less (14 %), than the value found on the original fracture surface (3@ %). Thus, we c o n c l u d e that the wide d i s t r i b u t i o n on the fracture surface m e a n s real physical d i f f e r e n c e s among the v a r i o u s fracture facets and thus real d i f f e r e n c e s b e t w e e n the two si,Jes are also present. This also m e a n s that for a number of GB facets the fracture d:~,~

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not d i s t r i b u t e the p h o s p h o r u s evenly. The same c o n c l u s i o n can also be reached by c o n s i d e r i n g the GB c o v e r a g e itself. We can get the GB coverage of p h o s p h o r u s by adding the second and third c o l u m n s of Table i, w h i c h are shown in the second column of Table 2. In the last two rows again the mean and s t a n d a r d d e v i a t i o n are given. The s t a n d a r d deviation, is lower than on the fracture surface leading to the same c o n c l u s i o n as before. It is evident that in the case of u n e v e n p h o s p h o r u s d i s t r i b u t i o n the t r a d i t i o n a l approach, that is the m e a s u r e m e n t of the c o v e r a g e on one side of fracture, cannot be used. The error made is given in the third column of Table 2 w h i c h c o n t a i n s the d i f f e r e n c e s b e t w e e n the double of the second c o l u m n of Table 1 and the s e c o n d column of Table 2 in percentage.

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Table 2: Peak height ratios =ox~erage~ d~ei~ion~ 1 18.4 15.0 2 26.6 -12.1 3 22.5 -16.3 4 21.1 19.8 5 20.7 -17.8 6 24.3 [email protected] 7 29.1 24.4 8 18.4 15.@ 9 28.@ 36.1 18 22.7 17.2 ii 28.5 -11.3 12 34.0 28.4 13 22.7 ii.i 14 15.8 33.4 15 23.8 -38.5 16 10.8 -19.6 mean std, dev(~)

23.@ 23.9

C~nelusions The fracture did not always e v e n l y d i s t r i b u t e the p h o s p h o r u s b e t w e e n the two newly c r e a t e d surfaces. This m e a n s that both the v a r i a b i l i t y of the coverages of the s e g r e g a t e d element m e a s u r e d on one of the fracture surface is larger than that on the GB, and that the GB coverage d e t e r m i n e d by the traditional m e t h o d is rather inaccurate. The GB c o v e r a g e s d e t e r m i n e d as the sum of the c o v e r a g e s on the two sides also v a r i e s from facet to facet, showing that the level of s e g r e g a t i o n changes with GB structure. A=knQwl~dg~m~n~ M. M. made the m e a s u r e m e n t s r e p o r t e d in this paper, while working with the D e p a r t m e n t of the M a t e r i a l s S c i e n c e s and E n g i n e e r i n g of the U n i v e r s i t y of Pennsylvania. Support for that w o r k was partly p r o v i d e d by the NSF MRL p r o g r a m under grant number 85-19059.

R~£e~ene.ea I. 2. 3. 4. 5. 6. 7. 8. 9. I@. ii.

12. 13. 14. 15. 16.

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